An average human eye can accommodate its focus within an object distance range of 7 cm to infinity—an average human eye, in other words, has difficulty focusing well on objects less than 7 cm away. Ideally, the object distance ranges from a couple of meters to infinity to prevent eye strain during prolonged viewing. Head mounted displays (HMDs) have thus been traditionally designed to project the image from an electronic display, such as a liquid crystal display (LCD) or an array of light emitting diodes (LED), through a lens and into the eye such that the image of the display is virtually projected at a distance far greater than the physical display distance. This prevents the HMD from being huge and ungainly.
Using this technique traditional HMD designs involve the use of a display, a collimator, and a method of relaying this collimated image into the eye, such as a waveguide. Some designs combine the collimator and the relay into one optical component, which can greatly simplify the design. But encapsulation of such designs into an eyeglass lens is often difficult because of size, geometry, and see-through constraints. In the interest of having a discreet HMD design, it would be ideal if an image could be projected into the eye without the use of bulky optics and micro-displays, while at the same time being see through and able to be encapsulated in the lens.